定义：一种光束合束的方式，需要合束的光束是相干的。

相干合束是一种通过光束合束进行功率放大的技术。目的是将多个高功率激光光束合在一起得到一束光束，不仅具有更高的光功率还保持光束质量，因此光束亮度提高。相干合束还保留了光谱宽度。另一种方法是光谱合束。 

并排合束与连续孔径技术比较 相干合束技术可以分为以下几种： 

图1：相干合束技术。a)并排合束的例子，其中四个光纤放大器的输出光结合成一束具有更大的光束面积。b)连续孔径合束的例子，采用了一个衍射光栅。 连续孔径合束的一个特殊的离子是相干偏振合束。其中，入射光束只能有两个。但是，由于输出光偏振态为线偏振，因此可以重复这一合束过程。 并排合束技术的发展受益于早期在相控阵天线和微波发射器和接收器的应用。在光学领域，由于采用的波长更短，因此实现合束非常困难，这需要非常严格的机械允差。 

任意情况下，都需要合在一起的光束是相干的，且相对相移需要小于1 rad r.m.s. 这在之前给出的两个例子中都提到了，在实际情况下似乎并不理想，但是理论上比较简单： 

除了相位相干，光束还需要具有稳定的线偏振态，幅值涨落也不能太大。 

得到相干光束的方法 有很多种技术可以得到相干光束，总结如下： 

非单色光相干合束 相干合束大多数情况下采用的是单色光。但是，入射光也可以是非单色光，只要是相干的即可。例如，具有很宽光谱的超短脉冲可以相干合成。这需要光程严格匹配，这样入射光束的峰值与输出光的峰值在相同的时间达到。光波带宽越宽，对于时延匹配要求越严格。对于非常宽带的脉冲（不考虑其时间延伸与否），臂长差在几微米或者更短是可以接受的。

还有一种被动合束的技术，其中入射激光能够自动的相干振荡，即使它们不是单频激光器。但是，主动稳定激光器阵列通常需要单频工作。 

一般性说明 总的来说，相干合束的技术并没有很广泛的应用，尽管很多方案都在进行研究。最大的困难是得到很高功率的足够稳定的相位相干的光束，不仅能够工作在实验室环境还可以在更噪杂的工业环境中应用。另一个问题是需要波前和偏振方向严格匹配和稳定。采用单频信号和高功率光纤放大器的方案需要采用额外的方式来抑制受激布里渊散射的产生的问题。在并排合束系统中，填充因子小于1也会降低光束质量，在远场光束中形成旁瓣。与之形成对比的是，采用光谱合束的系统对各种因子要求没有那么严格，但是如果需要窄的辐射光谱时需要相干合束。

Definition: a class of methods for beam combining, requiring mutual coherence of the combined beams

Alternative term: coherent beam addition

The term coherent beam combining (also called coherent beam addition) denotes one class of techniques within the more general technique of power scaling by beam combining. The goal is to combine several high-power laser beams so as to obtain a single beam not only with correspondingly higher power but also with more or less preserved beam quality and thus with increased radiance (brightness). Coherent combining also preserves the spectral bandwidth. An alternative class of techniques, discussed in a separate article, is spectral beam combining.

Techniques of coherent beam combining can be subdivided into

Figure 1: Techniques for coherent beam combining. a) Example of tiled-aperture combining, where the outputs from four fiber amplifiers are combined into one beam with a large area. Due to the synchronized optical phases, the overall beam has a reduced beam divergence. b) Example for filled-aperture combining, using a diffraction grating.

A special case of a filled-aperture technique is coherent polarization beam combining [21]. Here, one can have only two input beams. However, as the output polarization is linear, one can repeat the combination as required.

Techniques for side-by-side combining can be implemented with fiber arrays, for example. They may have been inspired by the earlier implementation of phased-array antennas in radio frequency and microwave transmitters and receivers. In the optical domain, the realization is more difficult due to the much smaller wavelength, which introduces correspondingly tighter mechanical tolerances.

In any case, mutual coherence of the combined beams is essential; typically the relative phase deviations need to be well below 1 rad r.m.s. This is illustrated for both mentioned sub-classes with two examples, which may not be ideal in a practical sense but conceptually simple:

Apart from phase coherence, the beams involved must have a stable linear polarization, and the amplitude fluctuations should also not be excessive.

There are a variety of techniques to obtain the mutual coherence, which are briefly summarized in the following:

There are also techniques of passive beam combining where the input lasers automatically get into mutually coherent oscillation (via some feedback or nonlinear interactions) even though they not single-frequency lasers. However, single-frequency operation is typically required for actively stabilized laser arrays.

A special case is the non-collinear coherent superposition of ultrashort pulse beams [25]. Here, the term beam combining should actually not be used, since one only exploits the superposition of two clearly separate beams in the region around their foci. For some applications, that can be sufficient.

Coherent beam combining is mostly done with monochromatic waves. However, it can also be applied to non-monochromatic input beams, as long as these are mutually coherent. For example, ultrashort pulses, having a broad optical spectrum, can be coherent combined [22]. It is then required that the path lengths are exactly matched such that the temporal peaks of the contributions of all input beams to the output occur at the same time. The broader the optical bandwidth, the more critical is that delay matching. For very broadband pulses (disregarding whether they are temporally stretched or not), arm length differences of only a few micrometers or even less are acceptable.

Overall, methods for coherent beam combining have not been very successfully applied, although many different approaches have been investigated. The main difficulty is to obtain phase coherence at high power levels in a sufficiently stable manner, working not only in a quiet laboratory environment but also in a mechanically more noisy industrial setting. Another challenge is the need to match precisely and stably wavefronts and polarization directions. Schemes using single-frequency signals and high-power fiber amplifiers may require additional measures to suppress problems with stimulated Brillouin scattering (SBS). In tiled-aperture systems, some degradation of beam quality is caused by a fill factor of less than unity, which leads to side lobes in the far-field beam pattern. In comparison, systems relying on spectral beam combining are more tolerant in various respects, but coherent combining may be used e.g. if a narrow emission spectrum is required.